1. Field of the Invention
The present invention is a device for use in measuring a process variable, such as level, pressure or proximity, based upon the changes in capacitance through the material measured. Changes in capacitance in a sensor are transformed to electrical current, the magnitude of which is indicative of the magnitude of the process variable being measured. Also, the measurement and reference outputs are used in a microprocessor which compensates for variations in dielectric constant in the measured medium by preprogrammed compensation equations. Calibration is automatically performed with a manual input to the microprocessor to vary the association of particular relative levels therein in association with specific output levels. That is, selected percentages of full scale can be associated with specific output electrical current levels from the sensor.
2. Description of the Prior Art
Capacitance to current transmitters have been used for many years for the detection of changes in process variables such as pressure of gases, the level of liquids and solids or the proximity of a body. Usually the transmitters employ a probe in the form of a rigid, rod-shaped electrode. The capacitance between the probe and a grounded terminal varies in response to the environment surrounding the probe. That is, for example, the capacitance changes as the liquid level rises in a vessel, since the liquid immerses an increasing length of the electrode probe.
Some of the prior capacitance transmitter circuits have included the variable capacitance of a sensor in an oscillating circuit, whereby the output frequency is a function of the variable sensor capacitance. U.S. Pat. Nos. 3,518,537 and 3,896,374 describe such circuits. Other circuits have used comparators for determining the difference between the charging rates of a standard capacitor and of the variable capacitance between the probe and a ground plane electrode. U.S. Pat. Nos. 3,487,300; 3,811,051 and RE 27,829 describe such systems.